1. Field of the Invention
The invention relates to a developing apparatus for developing an electrostatic image formed on an image bearing member with a developer, and more particularly to a developing apparatus of a mono-component developing type having a developer carrying member urged against an image bearing member.
Also, as the image bearing member, use can be made, for example, of an electrophotographic photosensitive member, an electrostatic recording dielectric member or the like, and the developing apparatus can be provided in a cartridge detachably mountable on an image recording apparatus (image forming apparatus) such as a copying machine or a printer, or an image forming apparatus main body.
2. Description of the Related Art
For example, in an electrophotographic image forming apparatus, (1) a nonmagnetic contact developing type and (2) a magnetic non-contact developing type are widely used as conventional mono-component developing types for developing an electrostatic latent image formed on an electrophotographic photosensitive member as a member to be developed (image bearing member) with a mono-component developer.
(1) Nonmagnetic Contact Developing Type
There has been proposed a type in which a nonmagnetic developer is carried on a developing roller (developer carrying member) having a dielectric material layer and is brought into contact with the surface of a photosensitive member to thereby effect development (see, for example, Japanese Patent Application Laid-open No. 2001-92201). The developer in a developing apparatus is supplied to the vicinity of the developing roller by a mechanical agitating mechanism or gravity. An elastic roller for contacting with the developing roller is provided and effects the conveyance and supply of the developer. This elastic roller also has the function of once removing any developer not shifted to the photosensitive member, but remaining on the developing roller, for the purpose of uniformizing the developer on the developing roller. A DC bias is applied to between the base material of the photosensitive member and the developing roller.
(2) Magnetic Non-Contact Developing Type
This type (see, for example, Japanese Patent Application Laid-open No. S54-43027 and Japanese Patent Application S55-18656) uses a magnetic mono-component developer, and carries the developer on a developing sleeve (developer carrying member) containing a magnet therein, and opposes the developing sleeve to a photosensitive member with a predetermined minute gap kept from the surface of the developing sleeve, and effects development by the developer flying in this gap. The developer in a developing apparatus is conveyed to the developing sleeve by a mechanical agitating mechanism or gravity and also, the developer receives a constant magnetic force by the magnet and is supplied to the developing sleeve. Then, a predetermined developer layer is formed on the developing sleeve by regulating means, and is used for development. The force acting on the developer by the magnet is positively used not only for the conveyance of the developer, but also in a developing portion. In the developing portion, the developer is prevented from shifting to a non-image portion to thereby cause a faulty image such as fog. This is because during development, the developer travels toward the magnet contained in the developing sleeve and receives the magnetic force. For the flight of the developer, use is made of a bias comprising an AC bias superimposed on a DC bias. The DC bias voltage is adjusted to a value between the image portion potential and non-image portion potential of the photosensitive member. Further, an AC voltage is superimposed, and the developer is reciprocally moved to the image portion and the non-image portion, whereby the image portion is developed with the developer.
(3) Cleaner-Less (Toner Recycle) System
From the viewpoints of the simplification of an apparatus construction and the elimination of waste, in an image forming apparatus of a transfer type, there has been proposed an electrophotographic process which disuses an exclusive drum cleaner which is surface cleaning means for a photosensitive member after the transferring step, and recycles a toner in the apparatus. For example, there has been proposed an image forming apparatus which uses the aforedescribed nonmagnetic contact developing type to collect any developer untransferred and residual simultaneously with the time of development (see, for example, Japanese Patent Application Laid-open No. H03-4276).
There has also been proposed an image forming apparatus which uses the aforedescribed magnetic non-contact developing type to collect any developer untransferred and residual simultaneously with the time of development (see, for example, Japanese Patent Application Laid-open No. H10-307455).
In the conventional nonmagnetic contact developing type mentioned under item (1) above, a reduction in the fog performance during endurance has been a problem. The characteristic of the toner is lowered while the mechanical stripping-off by the elastic roller is repeated, and the fog is sometimes aggravated by the lowering of the frictional charging characteristic or the like of the toner. The fog refers to the image fault that the toner is slightly developed in a blank portion (unexposed portion) which is originally not printed and appears like a ground stain. For the prevention of the lowering of the toner characteristic, it is also possible to weaken the frictionally contacting force of the elastic roller, but the compatibility with a ghost image fault is difficult. Here, the ghost image is the phenomenon that in a halftone image wherein the hysteresis of a toner amount developed in the previous rotation of the developing roller is uniform in the next and subsequent rotations, uneven image density appears with the phase difference of the outer periphery of the developing roller. Also, the presence of the ghost means that there is some toner which is not stripped off but is residual on the developing roller.
That is, the toner continuously receives the frictional contact by the elastic roller and this is not preferable also from the viewpoint of the lowering of the characteristic of the toner. The adjustment of the frictionally contacting force has the problem of not only being contrary from the viewpoints of fog and ghost, but also being contrary in the problem of fog singly.
Also, when the toner characteristic becomes lowered, there has also arisen the problem that the toner is liable to be affected by the circulation thereof in a developing device. Specifically, in mechanical circulation or circulation using gravity, there is formed an area in which the agent (the developer or the toner) hardly changes places and does not circulate particularly around the developing roller. On the other hand, the predetermined lowering of the characteristic occurs to the circulating agent. Thus, when the toner in a container has been decreased, the two kinds of agents are mixed together to thereby cause condensation or the like, and has given rise to the problem of fog or the like. Further, there is an image fault attributable to the elastic roller itself.
On the other hand, in the magnetic non-contact developing type under item (2) above, there is an image fault due to a magnetic ear. There is the problem that the uniformity of a hair line differs lengthwise and breadthwise. When the magnetic ear develops while moving in parallelism to the movement direction of a photosensitive member (photosensitive drum), the uniformity of the hair line is good and is liable to break off in a direction orthogonal thereto. Also, an image edge fault is caused. The edge of a high image density portion, particularly the downstream side of the process is developed darkly, and the edge of a halftone portion adjacent to the high image density portion is developed lightly. The factor is expected to reside in developing in non-contact while reciprocally moving the developer by an AC electric field (FIGS. 9A and 9B of the accompanying drawings). In the developing portion, the toner is moved toward a surface and particularly, the toner stagnates downstream of the edge portion and conversely, the toner is drawn near from the outside of the edge to thereby cause the image fault as described above. Further, in the image forming apparatus of the cleaner-less system, because of non-contact, the capability of collecting the toner on the photosensitive drum is low, and this leads to the problem that the untransferred residual toner becomes a ghost and appears in solid white or a halftone. Also, a white spot occurs in solid black. This white spot is liable to occur when paper dust gets mixed between the developing roller and the photosensitive drum under a high temperature and high humidity. This is expected to be because bias leak has occurred between the developing roller and the photosensitive drum with a result that the potential of the latent image on the photosensitive drum has risen (to the negative).